[lug] my "no-data-movement" file-copy attempt

Zan Lynx zlynx at acm.org
Tue Oct 18 09:34:04 MDT 2011

On 10/18/2011 1:33 AM, Doug Pintar wrote:
> I have two Western Digital
> SATA hard drives, primary being a WD5000AADS, secondary a WD2500JS. 
> These are both 7200 RPM drives; the stated "maximum internal transfer
> rate" for the 500 GB drive is 108 MB/sec (and this is probably
> drive-manufacturer decimal MB rather than MiB) and the 250 GB drive is
> 93.5 MB/sec.  I see a Linux transfer rate on the big disk of about 95
> MiB/sec, which is believable.  On the 250, however, I can't get it to go
> faster than about 58 MiB/sec.  This is independent of read or write
> direction.  You'd think, given the specs, that the numbers would be a
> lot closer.

Internal transfer rates are for just the drive electronics. It's usually
the rate it can handle reading and writing from the cache on the drive.

So the internal rate has nothing much to do with the rate that the drive
can read from the platters.

Beyond that, I am not surprised that a 250 GB drive is half as fast as
the 500 GB drive. One hard drive performance parameter is the density of
data on a track. A drive with double the data density will have double
the transfer rate, if the spin rate and track spacing are the same.

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